1. Field of the Invention
The present invention generally relates to multi-layer circuit structures and, more particularly, to the formation of connections between layers and at the surfaces of thermoplastic substrate layers and assemblies thereof.
2. Description of the Prior Art
As electronic circuits have become more complex, it has often been the practice in the fabrication of electronic devices and components thereof to assemble components on generally planar structures and to provide connections between components on the surface of or within the generally planar structure. Early examples of such generally planar structures were printed circuit boards having connections on one or both surfaces of a board made of an insulating material. With increased circuit complexity, particularly for digital circuits, multi-layer boards having connection patterns provided between thin insulating lamina and connections between conductive pattern layers formed at through holes became widely used.
As the demand for components including multiple integrated circuits or requiring circuits fabricated according to different semiconductor technologies has grown, other multi-layer structures have been used, such as multi-layer ceramic (MLC) or glass structures. These structures have also allowed an extremely high degree of miniaturization since each layer can be fabricated with extremely high accuracy. However, the formation of connections between layers has presented some difficulty. For example, in MLC devices, connection patterns are usually formed by screening of a conductive paste onto the surface of unfired layers of the ceramic material, generally referred to as green sheets. To make interlayer connections, the formation of holes in the green sheet was required as was the filling of the holes with conductive paste during screening. Such a procedure requires a minimum of two separate steps, each of which must be done with a high degree of accuracy of registration and would inherently result in some defective layers (requiring additional testing steps). More importantly, however, the green sheets with connection patterns thereon must be stacked with a high degree of accuracy and registration, subjected to pressure and sintered at a high temperature. This final high temperature operation effectively removes solvents and organic binders from the conductive paste (by means of which it was made of a screenable consistency) and results in connections which are porous, particularly in interlayer connection holes, known as vias. This porosity causes a reduction in effective cross-sectional area of the connection and increases resistance. Densification of the conductive paste has yielded some improvement in reliability of connections at the cost of additional processing steps. However, a substantial degree of porosity must remain inherent in the formation of conductors with a screenable paste since the constituent materials of the paste which permit it to flow, as in the screening process, must be removed from the final product. Since the porosity and distribution of voids in the via connections is not readily controllable, defects were often encountered in the course of manufacture or, later, in use.
For this reason, numerous techniques of producing via connections and various via connection structures have been proposed. For example, U. S. Pat. No. 4,346,516, to Yokouchi et al. discloses a technique of pressing soft conductive metal (e.g. gold) spheres into the soft, elastic material of the green sheets. However, as disclosed therein, the process was extremely critical as to the diameter of the spheres and subject to relocation or dislodging of the balls during the assembly of layers and sintering of the assembly, largely due to the resiliency of the green sheet and the dimensional changes of the green sheet during sintering. The process was further complicated by the contamination of the surfaces of the green sheets with particles of green sheet material and possibly conductive paste which was displaced as spheres were pressed into the green sheet material. Further, the structure of Yokouchi et al. is explicitly limited to connections made with single spheres, limiting the aspect ratio of the via connections and consequently limiting integration and connection density and imposing severe constraints on design rules as to feature size of screened conductive patterns. The formation of connections from the screened patterns to the spheres was unreliable due to the displacement of the conductive paste (which also was subject to defects due to porosity, as indicated above). Also, with spheres, especially in a single layer, the area of contact to the sphere is very small in relation to the volume of the spheres and cross-sections thereof. Further, the use of spheres, particularly if arranged to protrude slightly from the green sheet surface, often increased the difficulty of obtaining and/or maintaining accurate registration between layers and were particularly subject to being dislodged since retention in the green sheet is accomplished only through elastic deformation of the green sheet material. Registration of layers and the formation of interlayer connections is potentially made more difficult by dimensional changes during sintering of the green sheets, particularly in relation to the dimensions and dimensional changes of the conductive balls. These dimensional changes can also cause the balls to become dislodged or laterally displaced during sintering, as well. Additionally, the structure disclosed by Yokouchi is not applicable to the formation of pads for connection to integrated circuit chips which may be mounted on the MLC structure or engineering change external wiring.
More recently, thermoplastic materials of increased suitability for use as carriers for interconnection patterns have been developed. These materials generally are capable of withstanding higher temperatures with little or no dimensional change or tendency to spontaneously flow. So-called liquid crystal polymers, in particular, are very suitable for forming carriers for interconnection patterns since they are platable with metal to form a metallized layer which can be patterned by many known techniques such as selective deposition or etching. However, the reliable formation of interlayer connections through vias remains difficult at the level of current design rules for minimum feature size in multi-layer devices. Accurate formation of holes is difficult (although molding of holes provides some advantages in comparison to drilling as to both accuracy of location and number of process steps) and plating through vias is less than fully reliable particularly at hole aspect ratios (e.g. depth to diameter) of one or more. The use of a conductive paste is not desirable since it would require additional manufacturing steps and would not be of increased reliability in comparison to the use of such a paste in MLC devices. Screening of paste into holes of an aspect ratio of one or more is also not fully reliable. In addition, the use of thermoplastic materials for lamina in multi-layered structures is subject to misregistration as in the case of green sheets. Therefore, while lamina are readily moldable from thermoplastic materials, the need for the formation of via connections has not permitted the full exploitation of potential advantages of such materials in multi-layer electronic devices.